The present invention relates to an improved method for the explosive welding and expanding of tubes to a tube sheet, and more particularly, to an improved method for the explosive welding and expanding of tubes to bottom tube sheets of vertically mounted heat exchangers, such as high pressure feedwater heaters and nuclear steam generators.
Known methods for joining tubes to tube sheets, which typically are plates several inches thick through which holes are drilled for receiving the ends of the tubes, include the use of a packing between the tube and the tube sheet, mechanically rolling-in a tube using a rolling or expanding tool, and gas or arc welding, all of which for various reasons commonly lead to tube failures. Failures in welding methods, for example, can be related both to the tube end geometry (e.g., tube protrusion, weld rollover) and weld characteristics (e.g., porosity, heat affected zone, residual stress).
Disadvantages of known methods for joining tubes to a tube sheet, including large heat affected zones in conventional welding techniques and relatively higher residual stresses from mechanically "tormenting" the tube material in rolling-in methods, led to the use of explosive welding and expanding methods, an example of which is disclosed in U.S. Pat. No. 3,503,110 to Berry et al. In an explosive welding and expanding method, the explosive energy released when an explosive charge is detonated in the tube results in shock waves travelling through the tube which expand the tube within the tube sheet bore and create a metallurgical bond between the contact faces of the outer surface of the tube and the inner surface of the bore over at least a portion of the tube length located within the tube sheet bore.
The welding of the tube within the tube sheet, in this and other known welding methods, often results in a gap (or crevice) at the tube and tube sheet juncture, either at the back of the tube weld (when the explosive charge is selected so that no expansion results) or at the back of the tube expansion within the tube sheet, i.e., between the outer surface of the tube and the inner surface of the tube sheet bore. These gaps lead to accumulation of deposits and/or inadequate drainage in the areas surrounding the tubes and form sites for corrosion, particularly in vertically arranged units, because the accumulation of deposits cannot be adequately blown down or drained from the face of the tube sheet.
Proposed solutions to the aforementioned problem of gapping at the tube and tube sheet juncture are disclosed in U.S. Pat. Nos. 3,698,067 and 3,790,060, both to Feiss. These patents disclose an apparatus and method for mounting a tube in a perforated sheet by means of explosive pressure waves, with the stated object of eliminating gap corrosion between the tube and tube sheet. In one embodiment, a tube is inserted in a bore of a perforated end plate of a heat exchanger for the full thickness of the plate. The inserted tube is provided with two collars to ensure that it is disposed concentrically in the bore. A portion of small diameter between one of the collars and the end of the tube is to be metallically joined or welded to the end plate by a first explosive charge. A small diameter portion between the collars forms a uniform annular gap within the bore which is expanded over its entire length by a second explosive charge. The second collar, located near the position where the tube emerges from the tube sheet on the nesting side of the tube sheet, is intended to avoid slack between the tube and the tube sheet at the point where the tube emerges. The second collar also is intended to prevent inflation or bulging of the tube beyond the point at which the tube emerges from the tube sheet. In fact, preventing inflation or bulging of the tube beyond the tube face from which it emerges is a consistent object of the prior art explosive welding methods of which we are aware, as further demonstrated, for example, by U.S. Pat. No. 4,117,966 to Green et al. and U.S. Pat. No. 3,426,681 to Oliver.
Accordingly, it is among the objects of the present invention to provide an improved method for explosively welding a tube and tube sheet which eliminates the disadvantages of the known methods. In particular, it is among the objects of the present invention to eliminate the unacceptable gaps around the tubes at the juncture of the tube and tube sheet, but by a method which departs from the prior art in several significant respects, including the expansion of the tube beyond the rear face of the tube sheet from which it emerges as well as significant structural modifications to the rear face of the tube sheet.